One common method for printing images on a receiver member is referred to as electrography (also referred to as electrostatography). In this method, an electrostatic image may be formed on a dielectric member by uniformly charging the dielectric member and then discharging selected areas of the uniform charge to yield an image-wise electrostatic charge pattern. Such discharge is typically accomplished by exposing the uniformly charged dielectric member to actinic radiation provided by selectively activating particular light sources in an LED array or a laser device directed at the dielectric member (this embodiment is typically referred to as electrophotography). Alternatively, the image-wise electrostatic charge pattern may be formed directly on a chargeable member. After the image-wise charge pattern is formed, the pigmented (or in some instances, non-pigmented) marking particles, or toner, are given a charge, substantially opposite the charge pattern on the dielectric member and brought into the vicinity of the dielectric member so as to be attracted to the image-wise charge pattern to develop such pattern into a visible image.
Thereafter, a suitable receiver member (e.g., cut sheet of plain bond paper) is brought into juxtaposition with the marking particle developed image-wise charge pattern on the dielectric member. A suitable electric field is applied to transfer the marking particles to the receiver member in the image-wise pattern to form the desired print image on the receiver member. The receiver member is then removed from its operative association with the dielectric member and subjected to heat and/or pressure to permanently fix the marking particle print image to the receiver member. Plural marking particle images of, for example, different color particles respectively can be overlaid on one receiver member (before fixing) to form a multi-color print image on the receiver member.
These color printed images produced on electrographic devices have found many usage in both and commercial and consumer applications. Initially, the color print was primarily limited to marketing collaterals. However, a combination of sudden transformation of photography to digital format and improvements made in the quality of digital printing has placed additional constraints on electrophotographic prints that were not applicable for outputs from copiers. For example, it is not uncommon for consumers to have a photo albums printed using electrophotographic printers and use them in a manner they are accustomed to with traditional silver halide output. Examples of such cases would include leaving albums in the car on a hot humid day as well as leaving pictures and images in an attic where high temperatures and high humidity conditions often prevail.
As mentioned previously, a developed electrophotographic image on a substrate has to be fixed or fused before it can be properly handled. This requires a certain amount of melt flow of the toner when its temperature is increased as it is passed thru a fixing device. Examples of such fixing device include, a pair of heated rollers, radiant fusing, flash fusing, microwave fusing and the like. In all such cases, the melt flow of the toner should take place as quickly as possible upon heating. This can be achieved by selecting a low softening or glass transition temperature (Tg) of the toner binder and keeping molecular weight of the binder as low as possible. If the Tg of the toner binder is too high, then adequate polymer flow, necessary for proper fixing not achieved. Alternatively, higher Tg binder toners require a much higher fusing conditions, where the paper handling and life of the fuser rollers is drastically reduced. On the other hand, if the glass transition temperature of the toner is too low, then toner particles can fuse together either during shipping or inside a electrographic printers where the temperature is higher than the ambient. For this reason, useful range of the glass transition temperature for toner binder is typically limited to below 60° C., and typically from about 50 to less than 60° C. Also, when molecular weight of the toner polymer is too low, the toner tends to get very brittle which adversely affects the developer life and print physicals.
When electrographic images made with the toner binder as described above is subjected to high temperature, then print sticking becomes a common occurrence. During summer months, temperature of 55° C. and higher can be easily achieved in vehicles parked in the sun or in an attic. It is not very uncommon to have prints exposed to such condition as a result. With some polymers, relative humidity also becomes a critical factor. The reason for this is that many polymer binders are plasticized by moisture present in the atmosphere. Polyester, which is the most common type of toner binder used in the industry, is one such example of polymer, which can be plasticized by water. When a polymer is plasticized by water, its Tg is depressed by a certain amount proportional to the relative humidity in the atmosphere. As a consequence, a set of prints or a photo album can exhibit signs of sticking or “bricking” or “blocking” when they are subjected to such extreme temperature and relative humidity conditions. Such damage is irreversible and renders the prints useless. It is, therefore, highly desirable to have prints that can survive these extreme conditions.
Use of waxes and various other toner additives to improve fusing and other toner properties is known in the art. U.S. Pat. No. 5,783,348, e.g., discloses use of aliphatic amides and aliphatic acids as additives for providing low surface energy toners. U.S. Pat. No. 5,702,852 discloses use of aliphatic amide and aliphatic acids in non-marking toner to improve abrasion resistance. There is no suggestion in the prior art that use of selected aliphatic amides in a protective toner would uniquely improve blocking performance.